1. Field of the Invention
The present invention relates to a steering control system for a vehicle equipped with an electric power steering device, and in particular to such a steering control system for a vehicle which is capable of producing a reactive steering torque in response to a detected behavior of the vehicle.
2. Description of the Related Art
Power steering systems for reducing the steering effort required to steer the vehicle are known, and one example of such systems is disclosed in Japanese patent publication (kokoku) No. 50-33584. According to this power steering system, the steering torque applied from a steering wheel is assisted by an output torque of an electric motor whose magnitude primarily depends on the magnitude of the applied manual steering torque. By varying the gain for the detected steering torque applied to the steering wheel by the vehicle operator according to the vehicle speed and the road condition, the output torque of the assisting electric motor is adjusted so that an optimum steering assist torque may be obtained at all times.
When the vehicle encounters a strong crosswind or side wind, or runs into ruts or other irregular road surface features, the vehicle tends to deviate from the intended straight path of travel. When the vehicle travels over a low frictional coefficient road surface, such as a frozen road surface, or the vehicle is traveling at a low speed, the effort required to steer the vehicle significantly diminishes.
According to the conventional power steering system, however, the assisting torque is produced only when the vehicle operator has applied some steering torque to the steering wheel. Therefore, when the vehicle deviates from the intended straight path of travel due to crosswind, the electric motor will not produce any assisting torque by itself. Therefore, to prevent the deviation of the vehicle from the intended path, the vehicle operator must constantly apply a manual steering torque to the steering wheel. According to the conventional steering system, the assisting torque is generally small when the lateral acceleration and the yaw rate of the vehicle are large. Therefore, when the vehicle deviates from an intended path of travel, the steering input necessary to restore the vehicle to the intended path of travel becomes progressively greater as the lateral acceleration and the yaw rate of the vehicle increase.
It was previously proposed to compute a reference behavior of the vehicle in terms of a lateral acceleration and/or a yaw rate for a given steering angle and vehicle speed, detect the actual behavior of the vehicle, and apply a reactive steering torque so as to minimize the deviation of the detected vehicle behavior from the reference vehicle behavior. For details of such control arrangements, reference should be made to U.S. Pat. Nos. 5,528,497, 5,703,775, and 5,774,819 which are assigned to the common assignee, and the contents of these patents are hereby incorporated in this application by reference.
According to such a proposal, it is possible to control the vehicle in a stable fashion even when subjected to various external interferences. Also, the handling of the vehicle on a slippery road surface can be improved by preventing excessive steering. However, such control processes are based on the assumption that the vehicle is traveling forward, and would not work properly if the vehicle is traveling rearward. The vehicle speed sensor is typically unable to distinguish the traveling direction of the vehicle. As can be readily appreciated, the relationship between the steering angle and the resulting yaw rate when the vehicle is traveling rearward is reversed from that when the vehicle is traveling forward. When the vehicle traveling forward is steered to the right, the vehicle turns in clockwise direction as seen from above. However, when the vehicle traveling rearward is steered to the right, the vehicle turns in counter-clockwise direction, as seen from above. Therefore, when a reactive steering assist torque is produced when the vehicle is traveling rearward, it will act in the opposite direction and tends to produce an undesired result. For instance, when the vehicle is traveling on a low-frictional coefficient road surface, the reactive steering assist torque should increase the effort required to steer the vehicle so as to prevent any excessive steering. But, if the vehicle is traveling rearward, the reactive steering assist torque will reduce the effort required to steer the vehicle, and this promotes excessive steering to the discomfort of the vehicle operator.